REVERSE/THRUST FAULT The geology of the three major types of faults; reverse/thrust, normal faults and strike-slip faults: I. REVERSE/THRUST FAULTS - (general features) A. Compressional faults, layer parallel compression, they shorten and thicken the crust B. Thrust faults have low dips (generally < 20o) C. Result in formation of klippen and fensters D. The hanging wall of thrust faults is often called a "thrust sheet" or "thrust plate" E. Typically occur at the edge of folded mountain belts (e.g., Valley and Ridge province) F. Emplace older, deeper, higher-grade rocks upon younger, shallower, lower-grade rocks G. Two interpretations of thrust in layered sedimentary rock above a "basement" 1. Thin-skinned - faults level out at depth, and merge above the basement into a décollément. (e.g., Valley and Ridge) 2. Thick-skinned - faults extend below the layered sediments and cut the basement H. Geometry of thrust - ramp anticlines - Where the décollément breaks through along a ramp, bedding is bent along the ramp 1. The stratigraphic section is doubled in the circled area 2. The anticline only exists above the thrust 3. "hanging wall ramp" - where bedding above the thrust is truncated ( ) 4. "foot wall ramp" - where bedding below the thrust is truncated ( ) 5. Everywhere except for the HW and FW ramps, the fault is parallel to bedding 6. Dots are the same contacts, profile parallel to slip then retrodeformable if x>y=1>z (i.e., plane strain) I. Generally only the "cover" is shortened, the basement doesn't appear to be involved in the deformation in a particular area. But even where basement is involved, there is always too much cover. This is obvious in areas capable of being retrodeformed, where plane strain (x>y=1>z) has occurred and cross sections can be balanced. This is how the concept of subduction originated, "verschluckung", where the Aar massif was subducted beneath the Gotthard in the central Alps (Amferer, 1906, Jahrb. Geol. Bundesant. Austria, 56, p. 539-622); today the process of continental subduction is termed A-type subduction, in honor of Amferer, just as oceanic plate subduction is called B-type subduction for Benioff. II. BLIND THRUST A. Pressure solution cleavage B. The collapse and removal of CaCO3 and/or SiO2 due to dissolution 1. Insolubles - opaques, clays, muscovite, qtz 2. Domains = microlithons of undeformed material 3. Stylolitie - avenues of removal collapse pressure solution C. Indented grains D. Truncated fossils E. Pressure shadows F. Example N.Y., margin of Valley and Ridge III. TEAR FAULTS A. Above thrust (i.e., hanging wall) due to differential movements along thrust planes e.g., Pine Mtn. thrust KY and TN B. Tear faults in thrust sheets C. Lateral ramp IV. THRUST HORIZONS A. Are generally "bedding-plane faults" that form within lithologic units B. Shales and evaporites - commonly overpressured (high Pf) V. TIMING OF THRUST SEQUENCES A. Piggy-back thrust sequences - "foreland breaking" 1. Thrust get younger toward foreland B. Overstep thrust sequence - 1. Young toward hinterland C. Out-of-sequence thrust 1. Mixture of ages of thrusts VI. SYNTHETIC vs. ANTITHETIC COMPRESSIONAL OROGENS A. Conjugate shears: Burchfeil and Davis, 1968, I.G.C., Prague, Czech. p.175-184 VII. MECHANICAL PROBLEM WITH THRUST FAULTS (Smoluchowski, 1908) A. Force balance equation - sxx * z = tzx * x B. Byerlee equation - tzx = 0.6 * szz + 0.6 szz =rzg r = 2.5 gm/cc z = 5 km tzx = 1.34 kb szz = 1.23 kb z = 5 km sxx * z = tzx * x sxx * 5 = 1.34 * x x = 3.73 sxx sxx » 3-5 kb X max ~ 10-15 km width But, thrust sheets generally are > 50 km wide How do thrust sheets move? 1. Gravity sliding a. Gravity = BODY force (easier to slide entire BODY) tzx = 0.6szz + 0.6 kb , need larger angle to satisfy equation critical angle = 30o sin 30o = height / 50 km height = 29 km b. 29 km too high to move up thrust Answer: (pore pressure) in paper by Hubbert and Rubey, 1959, GSAB, p.115-166. 1. tzx = 0.6 szz + 0.6 2. szz = rgz 3. s = s - Pp 4. szz = Pp 5. szz < Pp 6. tzx = 0.6 kb 7. Overpressured shales and evaporites (gypsum = CaSO4 * 2H2O) D. Ray Price - (1974, in deJong and Scholten, eds., Gravity and Tectonics, Wiley and Sons, p. 491-502) E. Gravitational spreading - internal parts of orogens are spreading laterally due to gravity (plastic flow) F. Dave Elliot - (1976, J. Geophysical Research, v. 81, no. 5, p. 949-963) 1. It's not the slope of the fault surface that's important, it's the slope of the ground (Earth's) surface, called "The Critical Wedge Theory" 2. Like a bulldozer pushing a load of dirt